Compact integration of electrical power and semiconductor devices has wide-ranging applications, ranging from distributed sensor networks to cardiac pacemakers. One method of producing the required electrical power for such devices is through the direct, solid-state conversion of nuclear energy. This method, studied since the 1950's, requires a radiation source and a suitable semiconductor junction. Power sources that are based on these techniques are commonly called nuclear batteries, radioisotope batteries, radioactive batteries, or, in the case of using a beta emitting radioisotope, beta cells.
In order to achieve the desired device miniaturization required by many applications, one approach that has been studied is the fabrication of the radiation power source and the desired semiconductor device (e.g., IC) onto a single, common substrate. For example, U.S. Pat. No. 2,998,550 discloses a device in which a plurality of semiconductor-based devices (e.g., transistors, diodes) and a radioactive power supply are combined on a single semiconductor substrate. The disclosed device geometry provides electrical isolation of each of the semiconductor-based devices from adjacent devices. One such disclosed geometry provides a plurality of radial tooth-shaped members surrounding a central region wherein each of the tooth-shaped members is used for an individual semiconductor-based device while the radioactive power source is fabricated at the substrate's center. Proposed materials for the substrate include germanium, silicon, cadmium sulfide and indium antimonide.
U.S. Pat. No. 5,642,014 discloses a self-powered semiconductor device in which a radioactive power source and an IC are formed on a substrate, the substrate preferably of p-type material. The radioactive emitter is either fabricated directly into the power source's junction, for example by diffusing tritium atoms into a metal layer formed on the junction, or placed in immediate proximity to the source's junction. The use of a separate metal tritide layer provides some control over the radioactive exposure of the manufacturing environment.
Although co-fabricating a radioactive power source and an IC onto a single substrate offers advantages in size, this approach is not without problems. First, as the same substrate is used for both devices, significant limitations are imposed on the types of power sources and devices that can be fabricated as well as the manufacturing processes that can be used. For example, assuming the devices are silicon based, only low-energy radioisotopes can be used due to silicon's low threshold for damage under nuclear radiation. Second, unless the structure shields the IC from the radiation source, the device may become damaged or exhibit radiation-induced noise or false measurements.
Accordingly, what is needed in the art is a radiation powered device that does not suffer from the shortcomings of the prior art. The present invention provides such a device.